Abstract [en]

Abstract Interactions between metals and natural organic matter (NOM) are of great environmental importance and one of the key factors influencing hydrolysis, solubility, and speciation of the metals. However, studying geochemically relevant metals like Al, Fe, and Cu is sometimes associated with analytical problems; for example Fe and Cu are both redox active. Gallium (Ga) is a non-redox active metal that usually occurs at very low concentrations in environmental samples and therefore a wide concentration range of metal(III)–NOM species can be explored by adding Ga(III) to such samples. This makes Ga(III) a good probe and analogue for other metal ions, in particular Al. In addition, due to the increased usage of Ga in society, a better understanding of how Ga interacts with NOM is of importance but such studies are scarce. In this work, Ga(III) interactions with two different organic materials (Suwannee River natural organic matter and Suwannee River fulvic acid) were studied using infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy in a large experimental range (101–84,076 μg Ga g−1 dry weight; pH 3–8). Our IR spectroscopic results showed that Ga(III) is bonded mainly to carboxylic functional groups and suggested that only a fraction of the total number of carboxylic sites in the samples was actively involved in the bonding. Modeling of the EXAFS data revealed that Ga(III) formed mononuclear chelate complexes with NOM that strongly suppressed the hydrolysis and polymerization of Ga(III). At low Ga(III) concentrations (1675–16,649 μg g−1) organic complexes, consisting of 1–3 chelate ring structures, were the dominating species in the entire pH range while at higher concentrations (67,673–84,076 μg g−1, pH 3.0–7.0) we detected mixtures of mononuclear organic Ga(III) complexes, Ga(III) (hydr)oxide, and free Ga(III) (here defined as the hydrated Ga(III) ion and its soluble hydrolysis products). Moreover, the EXAFS results showed significantly higher contribution from second-shell C atoms (9–11) for the Ga(III)–organic complexes at the lowest concentration (101–125 μg g−1, pH 4.9–5.1), indicating formation of cage-like structures similar to Ga(III)–EDTA. Our combined results showed that Ga(III)–NOM interactions can be of importance for the solubility and speciation of Ga in environmental systems. Furthermore, the similarities between Ga(III) and previous Fe(III) results demonstrate that Ga(III) can be utilized as a probe for metal(III)–NOM interactions over an extended experimental range (e.g., pH and metal concentration) and thereby improve our knowledge about these interactions in general.

Hagvall, Kristoffer

Umeå University, Faculty of Science and Technology, Department of Chemistry.

2015 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

The fate and behavior of many metals in the environment are highly dependent on interactions with natural organic matter (NOM), which is abundant in most soils and surface waters. The complexation with NOM can influence the speciation of the metals by affecting their hydrolysis and solubility. This in turn will also have an effect on the mobility and potential toxicity of the metals. For aluminum (Al) these interactions are of high environmental importance since Al have been shown to have negative effects on plant growth, water living organisms, and fish.

This thesis will focus on the interactions between Al(III) and NOM in different environments and under varying geochemical conditions. To study this, infrared (IR) spectroscopy and X-ray absorption spectroscopy (XAS) have primarily been used. Due to the difficulties in analyzing Al using XAS, gallium(III), shown to be a suitable analogue for Al(III), was used as a probe to get complementary information from the Ga(III)-NOM system. The combined results from these studies showed that Ga(III) and Al(III) formed strong chelate complexes with carboxylic groups in NOM and that these complexes were strong enough to suppress the hydrolysis and polymerization of the metals. Furthermore, Al in organic soil and stream water samples was also studied using XAS and the results showed a variation in the speciation from a predominance of organically complexed Al(III) in the stream waters to a mixture of Al(III)-NOM complexes and precipitated Al phases (Al-hydroxides and/or Al-silicates) in the organic soils. To further study mineral-NOM interactions the effects of NOM on the dissolution of gibbsite (g-Al(OH)3(s); a common mineral in the environment) were investigated. The results showed that NOM can promote mineral dissolution and presence of inner-sphere Al(III)-NOM species on the gibbsite surface, detected by IR spectroscopy, could indicate a ligand induced dissolution. To further investigate the structure of the complex formed at the surface of the mineral, an EXAFS study was conducted on the ternary Ga(III)-NOM-gibbsite system. The results indicated either formation of inner-sphere complexes with Ga(III) acting like a bridge between NOM and the gibbsite surface, or the presence of two separate species; Ga(III)-NOM complexes in solution and a precipitated Ga(OH)3(s) phase.

As a sidetrack to the Al(III)-NOM studies, a new way of characterizing NOM was developed using simultaneous infrared and potentiometric titrations, multivariate data analysis, and chemical equilibrium modeling. An acid/base model for a fulvic acid was constructed, based on spectroscopic information about functional groups and their pKa values, and indicated that the fulvic acid is to be regarded as a tetra carboxylic acid consisting of at least four fractions of carboxylic acids. This demonstrates new possibilities to study the acid/base and metal complexing properties of NOM, in which the presence of carboxylic acid groups predominate, and to design equilibrium models more reliable than presented before.